much. >> thank you. >> thank you. >> >> woodruff: next, how the american education system stacks up in global rankings and the questions surrounding that assessment. jeffrey brown has the story. >> brown: it's considered by many the world's most important exam. the program for international student assessment, or "pisa" test, has been given to 15-year- olds in 65 countries every three years since 2000. a way to test and compare performances in reading, math and science. results from 2012 were released today, and, once again, the u.s. hovered near the middle of the pack, lagging in some areas even as other countries advanced. math remains the biggest challenge. 29 other systems had higher average scores than american high schoolers. the u.s. fared better in reading, where it ranked 20th, and in science, ranking 23rd. the best results were in east asia, where students from shanghai, singapore, south korea and japan, among others, placed near the top. pisa results also showed another concern for american teens as well: a smaller percentage of them reached the top levels of proficiency.